net: stmmac: Fix null-function call in ISR on stmmac1000

(resent due to overhelpful mail client corrupting patch)

At least on Meson GXBB, the CORE_IRQ_MTL_RX_OVERFLOW interrupt is thrown
with the stmmac1000 driver, which does not support set_rx_tail_ptr. With
this patch and the clock fixes, 1G ethernet works on ODROID-C2.
Signed-off-by: NMatt Corallo <git@bluematt.me>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -2804,7 +2804,7 @@ static irqreturn_t stmmac_interrupt(int irq, void *dev_id)
 				priv->tx_path_in_lpi_mode = true;
 			if (status & CORE_IRQ_TX_PATH_EXIT_LPI_MODE)
 				priv->tx_path_in_lpi_mode = false;
-			if (status & CORE_IRQ_MTL_RX_OVERFLOW)
+			if (status & CORE_IRQ_MTL_RX_OVERFLOW && priv->hw->dma->set_rx_tail_ptr)
 				priv->hw->dma->set_rx_tail_ptr(priv->ioaddr,
 							priv->rx_tail_addr,
 							STMMAC_CHAN0);